1. Field of the Invention
The present invention relates to the field of a bifilar helical antenna.
2. The Prior Arts
For wireless telecommunication, a bifilar helical antenna may generate a back-fire radiation suppose it is configured having a helix radius of 0.1 wavelength and a pitch angle of 10 degrees approximately, according to Nakano and Pattern. Such a back-fire beam of the bifilar helical antenna will be further split under the conditions of a helix radius of 0.1 wavelength and a pitch angle of 40 degrees according to Pattern's experiments, while the back-fire beam is circularly polarized under the conditions of a small radius and a pitch angle of 68 degrees approximately according to Nakano, in which the main beam will be moved laterally as the frequency goes higher.
Since a bifilar helical antenna resembles a single-arm helix antenna in structure, therefore, the phase difference between one arm and the next is almost constant when actuating a K-arm helix antenna, and the results of its analysis are about the same with an infinite-arm helix antenna. Based on this conclusion, a traveling-wave pattern multi-arm helix antenna can be designed by means of the same theory applied to a single-arm helix antenna.
In practical use, the frequency band of GPS is allocated at 1,575.42±1.023 MHz and 1,227.6±1.023 MHz, while that of the GLONASS is allocated at 1,598.0625 to 1,615.5 MHz and 1,242.9 to 1,256.5 MHz. For receiving satellite's signals, a receiver antenna must be an antenna having omni-directional field pattern and RHCP (Right-handed circular polarization) with respect to all possible coverage range of satellite, in which the wider the beam width of the antenna is extended, the more the number of satellite is covered.
Due to the criteria specified for minimizing volume and circular polarization, multi-channel, phase center, and antenna position are key factors to be considered when designing an antenna. The multi-channel interference is the main source of errors in GPS applications, and such interference due to low elevation angle can be minimized by using a large ground plane or introducing a null point in the field pattern of antenna at a low elevation angle. Nevertheless, as the RHCP signals coming from a GPS satellite will be changed into LHCP signals after being reflected by a ground surface or another object, it is possible for an antenna having a good axial ratio to receive the RHCP signals coming from a GPS satellite directly and receive negligible LHCP signals in case the reflected signals fall in the main beam of the GPS satellite.
Moreover, with regard to a bifilar helical antenna of U.S. Pat. No. 4,780,727, detailed instructions are required for mounting some collapsible components thereof, in which the quality of assembling and parameter trimming are extremely important to the antenna performance. Another dual-band helical antenna of U.S. Pat. No. 6,184,844 adopts multiple feed networks that results in a complicated manufacture process and an increased breakdown rate.
Besides, since the helix antenna possesses intrinsically broadband characteristics, it is necessary to take the bandwidth of input impedance of a feed network into account in the design of such antenna, so that the input impedance can be kept as almost a constant without affecting the antenna performance in the operation band. However, the operation mechanism of feed network mentioned in literature looks somewhat more complicated to increase the difficulties of fabrication process.
In addition to the feed network, a contingent design is required at a terminal of the helix antenna for the reason that a forward radiation can be generated by the reflection current to thereby affect the front-to-back ratio of the antenna field pattern. Documents have suggested some solutions, such as decreasing the reflection current by loading a resistor or improving the front-to-back ratio by providing a horn opening, in which, through the design of horn opening, the bandwidth of input impedance can also be widened in addition to the improvements of the front-to-back ratio. However, its operation mechanism is rather complicated and hence its fabrication process is more difficult, and that is the reason why the present invention is presented.